The year’s most important congressional hearing is at hand — not only because momentum in a presidential election is in play, but also because the legislative branch’s ability to conduct serious oversight is on the line.

On both fronts, the power to shape the public’s perception Thursday rests with Hillary Rodham Clinton. And, whatever else about her behavior and ideology remains open to passionate disagreement, this much looks clear: With a single glaring exception, she has made an exceptionally effective witness during her 31 previous appearances before Congress, dating back more than two decades. All those times testifying as first lady and then as secretary of State, along with eight years taking testimony as a senator from New York and even her time as a young House Judiciary Committee lawyer during the Watergate era, have provided Clinton with significant benefits in preparing for one of the biggest moments of her public life.

Panel Republicans are hinting they’re going to reveal important new evidence sure to put Clinton justifiably on the defensive. But her ability to rebut whatever gets put forward has been honed by all the preparation she’s had in the skills of parrying and posturing required for such a rarefied, must-see-TV hearing.

Her performance at the first Democratic presidential debate has been hailed by Clinton’s campaign team as a triumph borne of extensive practice in showcasing her expertise on the issues, her proactive talking points and her comebacks for all her opponents’ lines of attack.

And it’s a sure thing Clinton is devoting almost as much time readying for the Benghazi hearing — betting a bravura effort at the Capitol, just more than a week after her steely showing in the Las Vegas debate, would go a long way to quelling unease within the party about her remaining the front-runner for the 2016 nomination.

Such a one-two punch, her allies hope, would reduce the relevance of whatever decision Vice President Joseph R. Biden Jr. is making about running. No matter how crisply aggressive her defense, though, there’s no reason to believe Republicans will ever relent in their criticism of her handling of the Benghazi attack or her use of a private email for government business.

“I think it is pretty clear what their goal is,” she said with just a hint of an eye roll during the debate, “but I’ll be there, I’ll answer their questions.”

Clinton’s cause already got a significant boost, of course, from the extraordinary boast by House Majority Leader Kevin McCarthy that the GOP’s real goal in creating the panel was to cripple her candidacy.

“Everybody thought Hillary Clinton was unbeatable, right?” the California Republican said on Sept. 29 on Fox News. “But we put together a Benghazi special committee, a select committee. What are her numbers today? Her numbers are dropping. Why? Because she’s un-trustable. But no one would have known any of that had happened had we not fought and made that happen.”

McCarthy has since declared he misspoke and that the panel’s actual mission is the same as its official purpose when it was created 17 months (and almost $5 million) ago: To uncover the facts about the government’s behavior before, during and after the Sept. 11, 2012, terrorist attack on the U.S. consulate in Libya in which Ambassador J. Christopher Stevens and three other Americans died.

Even without McCarthy’s unwitting assist, which was buttressed last week by comments from GOP Rep. Richard Hanna of upstate New York, Clinton has built-in advantages from all the time she’s spent at witness tables on the Hill.

Her debut came in the last week of September 1993, when she testified before five different committees to explain and defend the Clinton administration’s plan for overhauling the health care system, which she’d helped formulate (largely in secret) from the first West Wing office ever assigned to a first lady.

Even Republicans dead set against the proposals professed themselves wowed by her understanding of the issues — she was never seen opening the leather folder of notes she carried — and her familiarity with the demographics and hospitals of lawmakers’ districts.

The proposal never got out of the legislative starting gate, but the appearances marked a high point for Clinton’s reputation as first lady, which soon enough sagged under the weight of GOP inquiries into her roles in her family’s Whitewater real estate venture and firings at the White House Travel Office.

She managed to avoid testifying before Congress on either those or other controversies for the rest of the 1990s, and so she did not appear as a witness again until her January 2009 confirmation hearing for secretary of State.

She sailed through that session despite some pointed GOP questioning about the possible conflict of interest between her new job and Bill Clinton’s global fundraising. Returning to the Hill three months later, she offered Republican opponents of abortion rights on House Foreign Affairs a forceful but even-keeled defense of Obama administration efforts to advance reproductive rights globally.

And she returned a score of additional times during the next four years — without creating any meaningful partisan dust-up — to talk with a half dozen different panels about Iraq, Afghanistan, foreign aid priorities, the State Department budget, diplomacy in the oceans and Russian nuclear weapons.

It wasn’t until her first testimony about Benghazi, just before leaving office in January, that she made national news by coming across as simultaneously cavalier and combative when pressed at Senate Foreign Relations about her lack of awareness about the potential for deadly violence there.

“The fact is we had four dead Americans,” she said. “Was it because of a protest or was it because of guys out for a walk one night who decided they’d go kill some Americans? What difference, at this point, does it make?”

It’s a defensive impatience Republicans on the Benghazi committee are sure to work to rejuvenate. Clinton’s long history of congressional hearing combat suggests she’s unlikely to make the same misstep again.